It is known to provide a rotary cutting tool having a cutting radius adjustable by a motor. However, the adjustment of the cutting radius is controlled by a control external of the tool. A dedicated form of connection between the spindle of the machine tool to which the tool is connected, and the tool itself is thus required to transmit the control signals to a motor, in order to cause the motor to effect adjustment of the tool. Such tools are not therefore retrofittable into the spindles of existing machines. Moreover machine tools with dedicated connections on the spindle are expensive to manufacture.